1. Field of the Invention
The present invention relates to a capacitor, and more particularly, the present invention relates to a trimmer capacitor in which fine adjustment of the capacitance can be made.
2. Description of the Related Art
As a conventional trimmer capacitor, there is, for example, a chip-type trimmer capacitor disclosed in Japanese Unexamined Patent Application Publication No. 6-20872. This trimmer capacitor, as shown in FIG. 5, includes a stator 31 that is integrally molded with a nearly semicircular fixed electrode 32 having anchor portions 40a and 40b disposed in the circumferential portion of the fixed electrode 32 using a resin 31a and a dielectric ceramic plate 33 defined by a ceramic plate 33a made of a dielectric material with a movable electrode 34. The upper surface of the stator 31 on which the fixed electrode is provided and the lower surface of the dielectric ceramic plate 33 on which no movable electrode 34 is provided are arranged to rotatably contact with each other. A metal adjustment shaft 35 is arranged to pass through a shaft hole 38 of the stator and a through-hole 39 of the dielectric ceramic plate 33. The tip portion of the metal adjustment shaft 35 and a metal spring (rotor spring washer) 36 are joined so that they mate with each other.
Then, in this trimmer capacitor, by rotating the dielectric ceramic plate 33 in a state in which the upper surface of the stator 31 is in contact with the lower surface of the dielectric ceramic plate 33 and by changing the area of the fixed electrode 32 opposed to the movable electrode 34, that is, the overlapping area, the amount of capacitance generated between the fixed electrode 32 and the movable electrode 34 can be adjusted.
However, in the trimmer capacitor as described above, after the fixed electrode has been integrally embedded in the resin 31, the flatness of the upper surface of the stator 31 and the surface of the fixed electrode 32 exposed over the stator is increased to improve the characteristics of the trimmer capacitor, and accordingly a process for polishing the upper surface of the stator 31 is generally required. However, in the above-described trimmer capacitor, since the stator 3 is molded using a resin and is soft, it is difficult to reliably and accurately perform the polishing, and by the process of polishing, there is in particular a problem that the fixed electrode may be removed. The removal of the fixed electrode 32 is caused by weak adhesion of the fixed electrode 32 to the resin 31a, although the fixed electrode 32 is embedded in the stator 31, and when polishing is performed while the fixed electrode 32b is removed, excessive polishing is performed, and as a result, a problem of a decrease in the capacitance of the trimmer capacitor occurs.
In the above-described trimmer capacitor, by embedding the anchor portions 40a and 40b provided on the fixed electrode 32 in addition to the lead-out portion 37 of the fixed electrode in the stator, the fixed electrode 32 is fixed at the three points and the mounting strength of the fixed electrode 32 to the stator 31 is increased.
Here, the anchor portions 40a and 40b are provided in the circumferential portion of the fixed electrode 32 and are nearly at a right angle relative to the surface of the fixed electrode 32, and the farther toward the tip portion, the larger is at least one of the width and the thickness. This is to decrease stray capacitance caused by the anchor portions and to prevent reduction of the bending strength of the stator 31.
However, in the above-described conventional trimmer capacitor, as shown in FIG. 5, the anchor portions 40a and 40b are disposed in the circumferential portion of the fixed electrode 32 and at a location where the line connecting the anchor portions 40a and 40b does not pass through the shaft hole 38 of the stator. Therefore, the tip portions 41a and 41b of the fixed electrode 32 are to be located outside a triangular area defined by connecting the anchor portions 40a and 40b and the lead-out portion 37 of the fixed electrode, and there is the possibility that the removal of the tip portions 41a and 41b of the fixed electrode cannot be prevented merely by affixing the fixed electrode 32 at the three points.
In order to solve this problem, a method of providing additional anchor portions at the tip portions 41a and 41b of the fixed electrode 32 may be considered. As a method for providing these anchor portions, there is, for example, a method disclosed in Japanese Unexamined Patent Application Publication No. 57-140731, and in this method, as shown in FIG. 6, a fixed electrode 51 is firmly affixed in a stator 53 by performing press working on the circumferential portion 52 of the fixed electrode 51 so that the fixed electrode 51 cuts into the resin material. A trimmer capacitor in which the method is used in the tip portions of a fixed electrode and anchor portions 50a and 50b are further provided to the tip portions 41a and 41b of the fixed electrode is shown in FIG. 7. In the trimmer capacitors in FIGS. 5 and 7, the same reference numerals are applied to equivalent portions. In the trimmer capacitor, because the anchor portions 50a and 50b are further disposed at the tip portions 41a and 41b of the fixed electrode, it is believed that the fixed electrode 32 is also firmly affixed at the tip portions to the stator 31 and that the floating of the fixed electrode 32 can be reliably prevented.
However, the trimmer capacitor in which the anchor portions 50a and 50b are disposed at the tip portions 41a and 41b of the fixed electrode by the above method has the following problem. FIG. 8 is an enlarged sectional view taken along line Y-Yxe2x80x2 of the fixed electrode 32 embedded in the molded stator 31 in FIG. 7, which has not been polished. As shown in the drawing, the anchor portion 50b extends obliquely downward from the surface 32a of the fixed electrode. Accordingly, in the process of forming the stator 31, when the upper surface of the stator 31 and the surface 32b of the fixed electrode embedded in the stator 31 are polished along line A-Axe2x80x2, the length of the fixed electrode 32 is extended in the direction of Y-Yxe2x80x2 by L, that is, the area of the fixed electrode 32 is increased. Here, as the anchor portions 50a and 50b are located in the direction in which the movable electrode 34 opposed to the anchor portions 50a and 50b rotates, the area of the fixed electrode 32 becomes larger in the direction of the anchor portions 50a and 50b, and this means that the area opposed to the movable electrode 34, that is, the overlapping area, becomes larger, and as a result, the problem of varied capacitance in the trimmer capacitor occurs. In the process of polishing stators, since it is difficult to maintain the amount of polishing exactly constant among stators, the capacitance varies due to the polishing, and accordingly, variations in the capacitance of each stator occurs.
In order to overcome the problems described above, preferred embodiments of the present invention provide a trimmer capacitor in which in the process of forming a stator, even if the upper surface of the stator is polished in order to increase the flatness of the upper surface of the stator and the surface of the fixed electrode embedded in the stator, the capacitance does not change, and the trimmer capacitor has stable characteristics.
According to one preferred embodiment of the present invention, a trimmer capacitor includes a stator on the upper surface of which a substantially semicircular fixed electrode is mounted, and a dielectric ceramic plate on the upper surface of which a movable electrode is mounted, the lower surface of the dielectric ceramic plate contacting with the upper surface of the stator, and by rotating the dielectric ceramic plate on the stator and changing the area of the fixed electrode opposing the movable electrode, the amount of capacitance generated between the fixed electrode and the movable electrode opposing each other through the dielectric ceramic plate is adjusted, and anchor portions are arranged so as to have a difference in level or to be stepped down from the surface of the fixed electrode and are embedded in the stator in the vicinity of the upper surface of the stator at the tip portions of the fixed electrode.
By arranging anchor portions so as to be stepped down from the surface of the fixed electrode, when the upper surface of a stator is polished in the process for forming the stator, the area of the fixed electrode does not change. Therefore, even if there are variations in the amount of polishing among stators, the area of the fixed electrode remains constant, and accordingly the area opposing the movable electrode, that is, the overlap portion is not altered, and the capacitance of the trimmer capacitor is maintained constant.
Furthermore, since the removal of the tip portions of a fixed electrode is prevented by arranging anchor portions at the tip portions of the fixed electrode, if excessive polishing is performed, the capacitance of the trimmer capacitor will not be reduced.
It is desirable that the tip portions of the fixed electrode have a cross-section that is exactly perpendicular or substantially perpendicular to the surface of the fixed electrode in the region extending from the surface of the fixed electrode to the anchor portions. In this way, the more accurate the cross section of the tip portion of a fixed electrode is, the more difficult it will be to change the area of the fixed electrode in the process of polishing the stators, and accordingly, it is possible to form an accurate trimmer capacitor in which the capacitance does not vary.
Furthermore, it is desirable that the anchor portion be arranged such that the tip portion and the anchor portion of the fixed electrode are supported by separate holding jigs and that the holding jig on the side of the anchor portion be moved substantially perpendicularly and in the opposite direction from the surface of the fixed electrode with respect to the holding jig on the side of the tip portion of the fixed electrode.
This is because anchor portions having a very accurate cross section in the region extending from a surface of the fixed electrode to the anchor portions at the tip portions of the fixed electrode can be formed by a simple method.
Anchor portions are further provided at the outer peripheral portion of the fixed electrode, and the anchor portions are substantially perpendicular to the surface of the fixed electrode and at least one of the width and the thickness thereof increases toward the tip portion.
In this way, by providing anchor portions in the circumferential portion in addition to the tip portions of a fixed electrode, the fixed electrode can be affixed in a stator at five points, i.e., the two anchor portions of the tip portion of the fixed electrode, the two anchor portions of the circumferential portion of the fixed electrode, and the lead-out portion of the fixed electrode. As a result, the mounting strength of the fixed electrode is further improved. Specifically, the fixed electrode is primarily affixed by three sections, i.e., the two anchor portions in the circumferential portion of the fixed electrode and the lead-out portion of the fixed electrode, and further by providing the anchor portions at the tip portions of the fixed electrode located outside a triangular area defined by connecting the three points, the tip portions of the fixed electrode are prevented from being lifted off.
Other features, elements, characteristics and advantages of the present invention will become apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.